Wall yield threshold and effective turgor in growing bean leaves

The rate of cell enlargement depends on cell-wall extensibility (m) and on the amount of turgor pressure (P) which exceeds the wall yield threshold (Y). The difference (P-Y) is the growth-effective turgor (P _e). Values of P, Y and P _ehave been measured in growing bean (Phaseolus vulgaris L.) leaves with an isopiestic psychrometer, using the stress-relaxation method to derive Y. When rapid leaf growth is initiated by light, P, Y and P _eall decrease. Thereafter, while the growth rate declines in maturing leaves, Y continues to decrease and P _eactually increases. These data confirm earlier results indicating that the changes in light-stimulated leaf growth rate are primarily controlled by changes in m, and not by changes in P _e. Seedlings incubated at 100% relative humidity have increased P, but this treatment does not increase growth rate. In some cases Y changes in parallel with P, so that P _eremains unchanged. These data point out the importance of determining P _e, rather than just P, when relating cell turgor to the growth rate.Abbreviations and symbols FC fusicoccin - m wall extensibility - P turgor pressure - P _e effective turgor - RH relative humidity - Y yield threshold - _w water potential - _s osmotic potential